Lateral resistivity measurement has been used for decades to measure high resistivity formations, e.g., tight sands and carbonates with no or few fractures or “vugs,” in the presence of low-resistivity drilling fluids. The ratio of formation resistivity to drilling fluids resistivity often exceeds 10000. Induction logging tools generally are unable to provide satisfactory measurement of such formations because the measurement may be highly biased by conductive mud. Electrode-based lateral resistivity measurement is also affected by conductive mud as electrical current tends to bypass the formation through the mud column. However, lateral resistivity tools minimize the electrical current bypass effect by forcing currents to enter the formation in a lateral direction.
Classical lateral resistivity tools employ a central measuring electrode to measure current leaving the electrode and a number of guard electrodes placed above and below the central measuring electrode. The guard electrodes help focus the current emitted from the measure electrode into the formation. Focusing current laterally into the formation is accomplished by minimizing the upward and downward current flows along the mud column around the central measure electrode. To do so, longitudinal potential drops above and below the central measuring electrode are monitored and the guard electrode currents are adjusted such that the potential drops at the monitoring electrodes are minimized. The depth of investigation is controlled by the total length of the guard electrodes or more precisely, the current focusing span. The longer the current focusing span, the deeper the depth of investigation generally is.
In logging while drilling, lateral resistivity measurements may be made by injecting electrical current to a drill collar. Because of the high conductivity of collar materials, current tends to leave the collar in radial directions, thus forming the current focusing effect. Meanwhile, at least one return electrode must be provided to collect the current returning from the formation. The location of the return electrode affects the depth of investigation of the measurement, whereas the length of the electrode affects the sensitivity of the measured resistivity to the formation in front of the electrode. Such sensitivity should be minimized so that the measured resistivity reflects only the resistivity of the formation directly in front of the measure electrode. To do so, a sufficiently long return electrode should be employed.
The prior art for lateral resistivity measurements while drilling employ either toroidal (e.g., doughnut-shaped) coils or electrodes. They have several shortcomings. Those tools employing toroidal coils require reducing the outer diameter of a drill collar at certain locations in order to build such coils, which may result in weakened locations on the drill collar. Second, the complexity of toroidal coils often substantially increase the construction and maintenance costs of the tools. A toroidal coil may include four major components: core material, antenna windings, cover or shield, and nonconducting filling materials. The shield, in particular, needs special care for designing and being locked to the collar. Moreover, toroidal coils must operate at a sufficiently high frequency, usually on the order of a few kHz, to induce a useful amount of current in the collar. The higher frequency raises the effective resistance of the drill collar and thus result in higher ohmic loss in the collar. The higher frequency can also reduce the depth of investigation of the measurement, especially in low-resistivity formations.
The primary shortcoming of the electrode-based apparatus is limitations on the size of an electrode that can be built on a drill collar. To mount an electrode to a drill collar, the electrode must be electrically insulated from the collar. This may be relatively easy for a small electrode (e.g., a few inches or less than a foot in length) such as those used for microresistivity measurement but will quickly become difficult or even impractical to do for a large electrode several feet long or longer. A long electrode is less durable in harsh downhole environments because the large areas of insulating materials inserted between the electrode and the drill collar may generally reduce the integrity of the drill collar and electrode. That is, integrity of the drill collar and electrode may decrease as electrode size increases. Second, electrodes mounted on the outer diameter of a drill collar can create pessimistic current paths that deteriorates or destroys the current focusing effect. This is illustrated in FIG. 1. In the figure, two guard electrodes 2 surround the measuring electrode 1 from above and below, respectively. All the electrodes are mounted about the outer diameter of the drill collar 10 and are electrically insulated 3 from the drill collar from the sides and underneath. It is often desirable to space the electrodes apart with collar materials in between for mounting purposes. In operation, current is usually injected to the guard and measure electrodes and returned to the return electrodes. Part of the injected current will enter, as desired, the formation in front of the guard and measure electrodes. Part of the injected current 5, however, will unfortunately leak directly to the return electrodes through the collar body between the electrodes. The current leakage deteriorates current focusing and lowers the ability of the tool to measure the formation resistivity in the presence of highly conductive mud. Accordingly, a downhole resistivity measurement tool overcoming shortcomings of the prior art is needed.
In one aspect, embodiments disclosed herein relate to a downhole tool used for measuring high-resistivity formations in the presence of drilling fluids including a body having a longitudinal axis and bore therethrough, an array of longitudinal electrode segments separated by electrical insulators, wherein substantially an entire cross section of said body comprises at least one electrode segment, at least one longitudinal electrode configured to emit a first electrical current into said formation and measure said first emitted current, at least one longitudinal electrode segment configured to emit a second electrical current for directing said first emitted current into said formation, and at least one longitudinal electrode segment configured to receive said first emitted current returning from said formation, wherein said electrical insulators facilitate measurement of said high-resistivity formations only after substantially all of said first emitted current being conducted between longitudinal electrode segments first passes substantially through said formation or drilling fluids or both.
In other aspects, embodiments disclosed herein relate to a method of measuring high-resistivity formations in the presence of drilling fluids, the method including providing a tool having an array of longitudinal electrode segments separated by electrical insulators, wherein substantially an entire cross section of said tool comprises at least one electrode segment, emitting and measuring a first electrical current from at least one of said longitudinal electrode segments, emitting a second electrical current from one of said longitudinal electrode segments for directing said first electrical current into said formation, and receiving said first emitted electrical current at one or more of said longitudinal electrode segments returning from said formation, wherein said electrical insulators facilitate measurement of said high-resistivity formations only after substantially all of said first electrical current being conducted between longitudinal electrode segments first passes through said formation or drilling fluids or both.